Principles of Silviculture Seeking More Benefits Than Ever Before from Their Rather Than Specifics of Application
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Silviculture — defined as the theory and practice of aged for multiple uses. In a woodland made up of two or controlling forest establishment, composition, and more stands, each one may be treated differently. This growth — is important to today’s landowners who are publication emphasizes the principles of silviculture seeking more benefits than ever before from their rather than specifics of application. The discussion woodlands. These benefits include water, wildlife, and should help the reader become more familiar with the recreation, as well as timber and other wood products. silvicultural terms used in forestry. Some additional Through the use of various silvicultural practices, these references are listed at the end of the unit. Other benefits can be increased and can make the woodland publications in this series cover certain silvicultural more useful. treatments that can be used to achieve specific Nature practices its own brand of silviculture, objectives. although such natural development of the forest may take hundreds or thousands of years. Nature, of course, THE ROLE OF CUTTING IN SILVICULTURE does not consult owners about preferences. Through The same tool that can be used to destroy a forest can silvicultural practices, a manager or owner can produce a also be used to build and improve one. In the past, this woodland that meets objectives and can do it faster than tool was the axe; today, it is usually the chainsaw. can Nature. In the forest ecosystem, literally every available space is occupied by some type of vegetation. The primary SILVICULTURE FOR IMPROVED BENEFITS way that the established forest can be altered or Silviculture involves developing a plan for influencing controlled is by killing trees and other plants. In most the forest to grow in a certain way in order to achieve cases trees are killed by cutting. If the trees have no certain goals. The range of silvicultural treatments commercial value, it is cheaper to kill them by applying possible depends on the biological requirements and a chemical herbicide or by girdling them. Girdling, growth characteristics of the tree species present, along which involves the removal or killing of a ring of bark with the characteristics of the site where they are around the tree to block the flow of food from the crown growing — including all of the physical, chemical, to the roots, causes the tree to starve and die. climatic, soil, and biological features of the area that Most silvicultural treatments have the objective of influence tree development and growth. Trees require promoting the growth of desirable species while water, nutrients, carbon dioxide, and sunlight to grow. discouraging less desirable species. In young forests, the Silvicultural practices regulate the availability of water, desirable trees are nurtured by removing the undesirable nutrients, and sunlight to selected trees. ones. In mature or overmature forests, old trees are A number of silvicultural treatments often can be used removed in order to provide room for new trees. From effectively for a given woodland. The treatment chosen the silvicultural perspective, cutting trees is the primary depends on the objectives and desires of the woodland method of establishing and tending forest stands to meet owner. It is important to realize that no single a woodland owner’s objectives. Cutting trees to obtain silvicultural treatment or scheme is best for all wood products and income may be of secondary woodlands and all owners. As long as biological con- importance. straints are respected, there often are many right ways to treat a stand silviculturally. Furthermore, primarily for INTERMEDIATE CUTTINGS sociological and economic reasons, no two landowners Any cuttings or treatments applied to established are likely to seek exactly the same set of benefits from immature forests for the purpose of improving them are their woodland. A woodland can be managed for a single referred to as intermediate cuttings and may be dominant use, or it may be man- undertaken for any of the following reasons: • To remove poor quality trees. They have medium-size crowns and, along with the • To remove weed tree species. dominants, they form the upper canopy of the forest. • To thin the stand and increase the growth rate of Intermediate trees are shorter than those of the preceding residual trees. two classes. Their crowns extend into the upper canopy • To free an understory of seedlings and saplings from layer; however, they receive very little light from above suppression by an overstory of larger, inferior quality and no light from the side. Suppressed or overtopped trees. trees are totally below the main canopy. They are the • To remove insect- or disease-infested trees. smallest trees and receive no direct sunlight. • To salvage timber damaged by insect, disease, or fire. Dominant and codominant trees grow fastest because they have large crowns that can absorb the sunlight Stand development needed for growth. Intermediate and suppressed trees In order to understand the biological basis for most grow more slowly and usually will not grow much faster intermediate treatments, it is helpful to know how a even if the forest is thinned to give them more sunlight. natural forest stand develops. Many forest stands in Such trees may be genetically inferior specimens, trees Pennsylvania originated during the early part of this that naturally grow slowly, or damaged trees. It is century, after repeated harvests for timber and fuel- important to note that these smaller trees in the lower wood. These harvests were in many cases followed by canopy are generally the same age as their faster- severe fires. Stands that originate over a relatively short growing, more vigorous associates. period of time are referred to as even-aged, that is, all the trees in such stands are of roughly the same age. During the early stages of stand development, following heavy cutting or other disturbances, thousands or tens of thousands of small trees may be growing on an acre. As the trees get larger, they begin to compete with one another for the limited amounts of light, moisture, and nutrients. Natural selection occurs and the number of trees gradually decreases (Table 1). At maturity, only 80 to 100 trees may be growing on an acre. The trees that die during this process eventually fall to the ground and slowly decompose. Crown position Growth in height is the most critical factor determining the survival of forest trees. As trees begin to compete with one another, the stronger trees forge ahead, grow taller, and develop large tops or crowns. Weaker trees are crowded by their competitors, and their crowns Shade tolerance become misshapen and restricted in size. This Tree species vary in their shade tolerance — their ability competition between trees for places in the forest canopy is described by differentiation of trees into crown to grow and reproduce in the shade of other trees. classes. Tolerant trees are those capable of growing in heavy Trees in a forest stand can be classified according to shade. Examples of tolerant trees are beech, sugar maple, the position and size of their crowns in relationship to and hemlock. Other species such as black cherry, paper the others in the stand (see figure). Dominant trees are birch, or tulip poplar require full sunlight in order to the largest trees in a stand. They have large, full crowns, grow properly; they are referred to as intolerant. Species and receive full sunlight from above and partial light such as red and white oak, red maple, and ash can grow from the sides. Codominant trees receive full light from in moderate shade and are said to have intermediate above but relatively little from the sides. tolerance. Table 1. Average size and number of trees per acre for a typical unmanaged, even-aged, mixed-oak forest at different ages. Age (years) 10 20 30 40 50 60 70 80 90 100 Trees per acre 3,200 1,800 930 505 342 262 215 187 150 90 Average dbh (inches) 1.2 2.4 4.0 5.9 7.5 8.8 9.9 10.7 12.2 14.8 2 Trees that are fittest from the standpoint of natural cutting, or salvage cutting, to describe different selection and thus destined to become dominant and intermediate treatments referred to collectively as TSI codominant in natural stands, are not always the best (Timber Stand Improvement). The principle underlying trees from the woodland owner’s point of view. all these treatments is the same: undesirable trees are cut Intermediate cuttings allow the woodland owner to and removed in order to promote the growth of the select which trees will remain and which will die. The desirable trees. A woodland owner’s objectives number of trees per acre is then reduced as it would be in determine which trees are desirable (trees to leave) and a natural stand, but trees that are desirable to the owner which ones are undesirable (trees to cut). Terms such as are favored while those that are less desirable are WSI (Wildlife Stand Improvement) or ASI (Aesthetic removed (Table 2). Stand Improvement) might better reflect the objectives of some owners. Table 2. Distribution of trees by diameter class before and after an improvement cut in a 70-year-old even-aged stand of oak-hickory. Dbh class Trees per acre SILVICULTURAL HARVESTING (inches) Before cut After cuts Tree In every forest stand, at some point all or some of the 4 12 12 0 trees reach maturity. As a tree becomes mature, height 6 24 8 16 and diameter growth decrease and eventually stop. More 8 12 2 10 branches die, wounds heal more slowly, and the tree’s 10 24 10 14 ability to resist damage by insects and disease decreases 12 30 10 20 considerably.